As it is known in the art, optical communication is a technology that uses glass or plastic threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages modulated onto light waves. In Dense Wavelength Division Multiplexing (DWDM) communication, fiber optic bandwidth is increased by combining and transmitting multiple signals simultaneously at different wavelengths on the same fiber. In effect, one fiber is transformed into multiple virtual fibers.
Endpoints in an optical network communicate over assigned channels. A wavelength, or frequency, is allocated to each channel and signals are forwarded end to end over the channel. Each portion of the frequency spectrum is allocated to different bands of communication. For example, communications in the Conventional band (C-band) modulate data onto light waves in the wavelength range of 1525 nm to 1565 nm. In order to fully utilize the bandwidth potential of each band, it is desirable to map as many channels as possible onto the band. For example, in practice the C-band is apportioned into sixteen communication channels at 200 Ghz channel spacing.
The frequency spacing between wavelengths in a communication band is defined by the spectral width of the laser. On newer components, it is fairly cheap to lower the spacing between channels because lasers are better. When communication channels are more closely spaced, the communication capacity of the band is increased.
However, one problem with using the full communication capacity of a band arises due to the gain characteristics introduced by components as a wavelength traverses from a transmit point to a receive point in a network. As the signal propagates from end to end it encounters a number of components, each of which add different amounts of ‘ripple’, or change in power value to the wavelength. As a result, at the receive endpoint in the communication path there is a a ripple in power and, therefore the Optical Signal to Noise Ratio (OSNR) of the received signal. Different components have different output gain characteristics depending upon the frequency of the input signal to the component. For example, an amplifier may have a high gain characteristic for wavelengths in a certain range of the frequency band, and lower gain characteristic for wavelength in a different range of the frequency band. Thus, in systems that desire to use the entire band, gain flattening filters must used with amplifiers to remove the undesirable gain characteristics from the amplified signals. However, gain flattening filters are expensive and undesirably increase the overall cost of the network.
One alternative is to use techniques such as Coarse Wavelength Division Multiplexing (CWDM) systems, wherein a small number of channels are mapped across a frequency band. CWDM systems carry less communication channels, and are used when this is appropriate. Thereby reducing the cost of the optical network due to less filtering and loss from the components. However, CWDM systems do not utilize the full communication potential of the bandwidth, and do not lend well to amplification due to the spectral width of the receive filters. It would be desirable to identify a low cost method for utilizing multiple frequency channels in a communication band on an amplified network.